The use of geosynchronous satellites to distribute television signals has revolutionized the television broadcasting industry and helped to make cable television distribution systems possible. As is well known, there are many communications satellites encircling the earth occupying so-called "geosynchronous orbits" (meaning the satellites appear to be stationary with respect to fixed points on the earth). These satellites receive television signals originating from the earth (so-called "uplink" signals) and retransmit those received signals back to earth (the retransmitted signals are called "downlink" signals) While satellites typically employ directional antennas to transmit the downlink signals, the high altitude of the satellites ensures that a large portion of the earth can receive the downlink signals. Thus, a single satellite can distribute television broadcasting signals to entire continents or to large portions of continents.
For television signals, the uplink and downlink frequency bands are divided into a plurality of channels or "transponders". Satellites operating in the so-called "C-band" (3700-4200 MHz) provide standardized 40 MHz channel spacing--thus providing a 500 MHz-wide band or block of frequencies defining 12 channels of a given polarization (horizontal or vertical). Newer C-band satellites provide staggered horizontally and vertically polarized channels (absolute channel spacing is only 20 MHz but adjacent channels of the same polarization are spaced 40 MHz apart) so that 24 different 40 MHz-wide channels are provided within the 500 MHz C-band satellite bandwidth. Satellite receiving antennas commonly provide feed horns and other related components capable of separating horizontally and vertically polarized signals--permitting receiving stations to separate the 12-channel block of horizontally polarized signals from the interleaved and overlapping 12-channel block of vertically polarized signals. Thus, the odd numbered channel transponders are typically transmitted on one polarity, and the even number transponders are transmitted on a polarity 90 degrees away from the given polarity A satellite antenna with a "dual polarity feed horn" typically provides two coaxial cable transmission line outputs --one cable carrying the odd (first polarity) transponder channels and the other cable carrying the even (second polarity) transponder channels.
The C-band nominal transponder frequencies are standardized so that a transponder for a given channel number will have the same nominal frequency regardless of which satellite is transmitting it. Some satellites also provide additional television signal transponders on the so-called "Ku-band" (11.8-12.3 GHz). This additional 500 MHz-wide band has not been standardized with respect to transponder center frequencies, however, so that channel spacing and channel polarization may vary from one satellite to another. To receive Ku-band signals, earth stations must include programmable "frequency agile" receivers that can receive the signals at virtually any center frequency within the Ku-band.
Since transponder frequency bands are uniform from one satellite to another, satellites are spaced in orbit relative to one another such that a directional earth-based satellite antenna may be aimed or "focused" on a single satellite at a time. Downlink signal levels received by earth receiving stations are extremely low in level--mandating the use of directional high gain receiving antennas (such as parabolic "dishes") to obtain sufficient received signal levels Thus, a typical satellite receiving antenna must be physically re-aimed to receive the signals from a different satellite. Thus, multiple antennas are required to receive signals from multiple satellites simultaneously. Typically, only a subset of transponders of a particular satellite may be active at any particular time, and only a subset of the active channels may be suitable or desirable for viewing. It is therefore typically important in most commercial multi-subscriber systems to provide multi-satellite receiving capability (e.g., by providing multiple fixed antennas aimed at different satellites) along with a capability to select only a subset of the received satellite transponder television signals for distribution to subscribers.
Some ku-band signals and all C-band signals both use standard 40 MHz wide (actually 36 MHz) channel bandwidths and other channel parameters. Satellite-transmitted television signals include a frequency modulated (FM) video signal and generally use a 6.2 (or 6.8) MHz audio subcarrier signal that is frequency modulated with the audio signal. This FM/FM format is very different from the conventional NTSC terrestrial television broadcasting signals transmitted by all domestic U.S. television broadcasting stations (such terrestrial signals include a vestigial sideband amplitude modulated video portion having approximately a 5.75 MHz bandwidth and a frequency modulated sound carrier for a total bandwidth of about 6 MHz). Standard VHF/UHF television receivers are therefore not directly compatible with satellite television transmissions, and additional signal processing is required to convert satellite transponder signals to a form receivable by a standard receiver. The signals received from satellite must either be demodulated and converted into standard NTSC format television signals for application to the tuner of a conventional television receiver; or the satellite signals may alternately be received and demodulated to provide baseband video and audio signals which may then be applied to baseband inputs of a studio type monitor and audio system or the like.
Generally, a home television viewer watching television signals transmitted by satellite obtains those signals either directly via a satellite receiving antenna in his yard, indirectly via a cable distribution network, or indirectly via a local VHF/UHF television station. Television stations typically receive network "feeds" via satellite receivers and retransmit the signals over normal VHF and/or UHF television channels in NTSC standard television broadcast format for reception by receivers.
In most urban areas, cable television companies supply NTSC television signals (some timers in scrambled format) over coaxial cables to subscribers, many of these signals (e.g., so-called "premium channels" such as HBO, SHOWTIME, etc. and so-called "national network" channels such as the Fox Television network, ESPN and the Turner Broadcasting Network) being obtained from satellites. The cable television company receives the signals from one or more satellites (typically via one or more fixed position high-gain satellite receiving antennas directed the appropriate satellites) and converts selected received signals to NTSC broadcast type AM signals at frequencies in the VHF/UHF frequency range for application to the cable distribution system. Decoder units installed at subscribers' homes generally shift the frequency of selected signals carried by the cable to a preselected standard VHF television broadcast channel frequency (e.g., 60-66 MHz corresponding to VHF television channel 3) for further demodulation by the subscribers' television receivers, and may also "descramble" certain cable signals (e.g., by re-inserting suppressed sync signals, suppressing interfering signals intentionally applied to the cable, or the like) in a well-known manner. The cable television "head end" typically provides appropriate frequency conversion of satellite-obtained television signals, locally generated television signals, and locally received television signals so that the distributed signals do not interfere with one another and so fall within appropriate frequency conversion bands or channels for selection by the subscriber decoders (or "cable ready" television receivers), and provide a generally contiguous block of occupied channels (so that each "channel" defined by the cable television decoder corresponds to an active signal--thus preventing subscribers from having to view channels carrying only noise when switching between active channels).
The following is a non-exhaustive but somewhat representative listing of prior patents and publications related to "cable television" and related television signal distribution techniques:
U.S. Pat. No. 4,530,008 to McVoy; PA0 U.S. Pat. No. 4,580,161 to Petrus; PA0 U.S. Pat. No. 4,558,358 to Onda; PA0 U.S. Pat. No. 4,066,966 to Takeuchi et al; PA0 U.S. Pat. No. 4,484,218 to Boland; PA0 U.S. Pat. No. 4,486,773 to Okubo; PA0 U.S. Pat. No. 4,538,174 to Gargini et al; PA0 U.S. Pat. No. 3,936,594 to Schubin et al; PA0 U.S. Pat. No. 4,183,054 to Patisaul; PA0 U.S. Pat. No. 4,395,734 to Rypkema; PA0 U.S. Pat. No. 4,512,033 to Schrock; PA0 U.S. Pat. No. 4,648,123 to Schrock; PA0 U.S. Pat. No. 4,513,315 to Dekker et al; and PA0 U.S. Pat. No. 4,532,543 to Groenewegen. PA0 U.S. Pat. No. 4,545,075 to Miller et al; PA0 U.S. Pat. No. 4,130,801 to Prygoff; PA0 U.S. Pat. No. 4,429,418 to Hooper; PA0 U.S. Pat. No. 4,556,988 to Yoshisato; PA0 U.S. Pat. No. 4,509,198 to Nagatomi; PA0 U.S. Pat. No. 4,538,175 to Balbes et al; PA0 U.S. Pat. No. 4,710,972 to Hayashi et al; PA0 U.S. Pat. No. 4,761,825 to Ma; PA0 U.S. Pat. No. 4,592,093 to Ouchi et al;
In more rural and remote areas where cable television is unavailable, people use entire "stand-alone" satellite receiving stations for receiving satellite television transmissions--typically providing a received television signal quality that far surpasses signal quality from cable television or reception of terrestrial signals. Such earth stations typically include a satellite receiving antenna ("dish") and associated motor-controlled positioning mount; a low noise amplifier ("LNA") located at the antenna for amplifying the weak signals received by the antenna; a LNA block converter stage ("LNB"; usually located at the antenna) for down-converting the block of transponders (channels) received from the satellite (typically this down-converting stage converts C-band signals from 3.7-4.2 GHz down to the 900 MHz-1500 MHz range and converts Ku-band signals to the same range) for conveyance from the antenna to inside the home over coaxial transmission lines; and a conventional "satellite receiver" which performs the channel selection and further frequency/mode conversion processing required to allow the user to view a selected television signal on a standard television receiver and/or video monitor.
Such satellite receivers have been commercially available for quite some time from a variety of different manufacturers such as, for example, Microdyne Corp. of Ocala, Fla. and Zenith Electronics of Glenview, Ill. A typical modern satellite receiver includes a programmable microprocessor and can receive and process any C-band or Ku-band satellite transponder accessible to domestic and commercial downlinks. The satellite receivers typically allow users to select frequency from front panel controls and can digitally store preset settings for multiple combinations of frequency, format, signal polarity and satellite. Many satellite receivers also include integrated Videocipher II decoders to provide conventional descrambling of scrambled satellite television transmissions.
The following is a non-exhaustive but somewhat representative listing of prior publications and patents related to satellite receivers and receiving techniques:
Konishi et al, "Satellite Broadcasting", 89 SMPTE Journal no. 3, pages 162-66 (March 1980);
Grant, "Direct Broadcast from Lower Power Satellites", 1981 Proceedings of the IEEE International Conference on Communications pp. 26.1.1 to 26.1.5 (June 1981);
Cooper, "How to Build a Satellite TV Receiver", Radio Electronics (1981); and
Douville, "A 12-GHz Low-Cost Earth Terminal for Direct TV Reception from Broadcast Satellites", IEEE Proceedings on Consumer Electronics (1977).
Miller et al cited above disclose a fiber optic link for carrying received signals from the antenna site to a remote satellite receiver. Briefly, block converters are used to down-convert the antenna LNA output to a lower frequency band. A wide-band optical link is used to carry this down-converted output to the remote satellite receiver. At the satellite receiver end, another block converter up-converts the block of signals to their original frequencies for application to the satellite receiver.
As mentioned above, satellite earth stations are capable of providing extremely high quality received signals--in part because of a phenomenon known as the "FM improvement factor." Briefly, noise effects on the received image disappear when the received carrier level is sufficient to cause the receiver input to limit (i.e., when the carrier "fully quiets" the receiver). That is, when a sufficient signal level arrives at the receiver to start limiting action, the receiver quiets--and the background noise entirely disappears. The carrier level required to fully quiet a satellite receiver depends upon the sensitivity of the receiver, but can typically be easily obtained with a parabolic receiving antenna of sufficient diameter equipped with a relatively inexpensive low noise amplifier. Full quieting results in a much larger effective signal-to-noise ratio at baseband frequencies than is actually provided by the system components at satellite downlink frequencies. In contrast, no such "FM improvement factor" phenomenon applies to terrestrial free-space television broadcasting (or to conventional cable television signals) because these signals are transmitted in the NTSC AM format.
One proposal advanced in the past to overcome noise problems in cable television involves converting the received television signals at the head-end to digital signals, distributing the digital signals to subscribers (over coaxial or optical links) and converting the distributed signals back to analog form at the subscriber end. See Patisaul et al and Dekker et al cited above. The Patisaul et al patent teaches distributing VSB digitally encoded television signals via an optical transmission link to subscribers. Dekker et al relates to transmitting digital audio signals received from satellites over a community television distribution system. However, such conversions would require a significant amount of customized equipment at the subscriber end--substantially increasing the overall system cost.
In addition, a few experimental systems have been proposed which use optical fibers instead of coax to help eliminate noise. For example, in October 1986 Genstar Southern Development of Orange County, Fla. announced that it would offer cable TV service via fiber optic cable to 1,300 homes in Florida. The proposed system included a "head end" that received the television signals through satellite receivers. A selector node, connected to the head end by a 48-fiber single-mode fiber optic cable, was to select the channels for customer. In the home, an optical network interface was to translate the optical signal and transmit it over coaxial cable to the television receiver.
As those active in this art appreciate, a significant niche in the market for television signal distribution systems relates to so-called "community television" systems. A community television system typically provides television service to a relatively small "community" of subscribers such as the residents of an apartment building or complex of townhouses or condominiums; the guests of a hotel or motel; or patients within a hospital. Often, aesthetic considerations, lack of space and other considerations prevent each resident from erecting his own VHF/UHF or satellite receiving antenna and moreover, cost and convenience considerations dictate that residents share an overall television distribution system rather than each purchasing and installing their own system. Condominium and townhome complexes often provide single coaxial cable "drops" from a central service point to each individual dwelling. While cable television companies sometimes make use of such preexisting cables to install cable television service on a subscription basis, it may in many cases benefit residents in terms of cost and signal quality to purchase and provide their own independent community television system. In the case of hotels, motels and hospitals, a significant profit can be realized by controlling the distribution of premium and non-premium television transmissions and offering those transmissions to guests/patients on a subscriber or pay-per-view basis.
In the past, such community television systems typically received and distributed signals from one or more VHF/UHF antenna installed on the building roof or on a tower. However, with the advent of satellite television and the recent decrease in the cost and wide availability of satellite receiving equipment, many community television systems have purchased satellite receiving antennas and associated earth station components. These community television systems typically receive the satellite signals, descramble the received signals if necessary, convert the received satellite transmissions into NTSC format and distribute the converted signals to viewers.
Despite intensive development effort expended on the cable and community television industry, much further improvement is possible. For example, typical community broadcasting systems offer only a limited selection of channels and received signal quality may be mediocre if only terrestrial broadcasts are being received. Cost considerations are almost always critical in these types of systems. It would be highly advantageous to provide an increased selection of channels at the same or better signal quality using less expensive equipment.
The present invention provides an improved satellite television signal receiving and distribution system incorporating some highly innovative concepts. The resulting system provides a number of advantages over past systems, including the following highly advantageous features:
Economic distribution of high-quality FM satellite signals at frequencies compatible with standard conventional satellite receivers (thus permitting subscribers to take advantage of the performance, quality and special features provided by stand-alone earth stations without requiring them to purchase and install expensive and possibly impractical satellite antennas);
Multiplexed distribution of many (e.g., 48) satellite transponders over single subscriber drop cables, thus providing full compatibility with prewired building complexes and saving cabling costs (normal satellite transmission formats generally prevent transmission of more than 12 transponders in the 950-1450 MHz down-converted C-band over a single cable);
The ability to distribute any selected transponders from any of multiple satellites and from either polarity--thus providing customized blocks of selected active channels from several satellites in an integrated manner;
Distribution of FM satellite transponder signals (as opposed to conventional distribution of AM NTSC signals) to provide superior picture and sound quality as well as full compatibility with standard mass-produced satellite receivers;
The ability to provide subscribers with pay-per-view capabilities economically and automatically using already existing techniques provided for satellite receiving earth stations;
The ability to pass high definition television (HDTV) signals directly to subscribers without system alteration;
The capability of combining C-band and Ku-band satellite signals on the same cable;
The ability to economically provide full subscriber addressability; and
The capability of correcting erroneous center frequencies of received satellite transponder signals prior to distributing the signals (such frequency errors can be caused by poor LNB down conversion or satellite transponder variations).
Conventional wisdom in the prior art was to convert received satellite signals into standard NTSC AM signal formats before distribution to permit subscribers' conventional television receivers to successfully demodulate the distributed signals. In accordance with one important aspect of the present invention, this conventional wisdom is entirely ignored. Instead of distributing AM television signals, the present invention provides distribution of a block of transponder signals each in the same form as they are received from a satellite downlink. The distributed FM/FM signals are completely incompatible with standard television signals, but are fully compatible with standard off-the-shelf satellite receivers designed for decoding/selecting signals obtained from a standard satellite receiving antenna/LNA/LNB arrangement. Thus, each subscriber simply uses a standard satellite receiver of the type designed for stand-alone satellite earth station receiving systems for selection of a particular transponder ("channel") and conversion of the transponder signal to either baseband (for viewing on a studio type video monitor) or NTSC AM format (for viewing on a standard television receiver).
Several advantages are obtained by distributing satellite transponder signals without converting the signals to NTSC (or some other) format. For example, standard off-the-shelf mass-produced satellite receiver units can be used for decoding/demodulating at subscriber locations--thus significantly decreasing system cost and complexity while increasing system reliability and simplifying inventory logistics. Using standard satellite receivers also permits the system provided by the present invention to take advantage of features already offered to stand-alone "TVRO" earth station owners (including subscriber-addressable signal descrambling using the standard Videocipher II system) and additional features that may become available to such earth station owners (e.g., "Video Pal" pay-per-view for use in connection with Videocipher II descrambling, and decoding of HDTV signals transmitted over satellite transponders). As will be appreciated, HDTV signals can be passed over existing satellite transponders (see, e.g., Jurgen, "Chasing Japan in the HDTV Race", 26 IEEE Spectrum No. 10, pp. 26-30 (October 1989).
The present invention, however, actually provides subscribers with additional features not typically available from a stand-alone TVRO earth station. For example, typical earth stations operated by individuals for their own use generally have only a single satellite receiving antenna which can be aimed at only a single satellite at a time (e.g., using a motorized drive) and which generally cannot simultaneously provide horizontal and vertically polarized signals (unless dual cables are provided from the antenna to the satellite receiver and the satellite receiver includes circuitry for selecting between polarities). Thus, a viewer using a typical earth station is limited to selecting from a maximum of 12 (or in some cases 24) transponders provided by a single satellite. Selecting the other polarization may be relatively easy, but selecting another satellite is typically more time-consuming (since the antenna must actually be physically redirected manually or using a motorized antenna positioner). Moreover, most satellites typically do not "fill" all 24 transponders with useful television signals all the time, and many of the signals transmitted by a particular satellite may be of no interest to the average viewer.
In accordance with a further aspect of the present invention, transponders from multiple satellites and/or multiple polarities may be "mapped" or converted into desired frequency transponder "slots" within the signals distributed to subscribers (see FIG. 2A). Hence, a 12-transponder block of channels presented to subscriber satellite receivers may contain signals from several different satellites and from both horizontal and vertical polarizations. In fact, the preferred embodiment system provided by the present invention is capable of "mapping" any transponder signal of any satellite into any desired transponder frequency (i.e., the same or different transponder frequency). Moreover, errors in transponder center frequency can be corrected through this mapping process so that the transponder signals distributed to subscribers require no "fine tuning" by the satellite receivers located at the subscriber end plural.
In accordance with a further feature of the present invention, plural 12 transponder blocks of signals are distributed to subscribers while requiring only single cable "drops" to each subscriber location. Specifically, plural distribution cables each carrying a 12-channel block of satellite transponder signals are routed to a centralized location such as a "wiring chase". A multiplexer/selector for each subscriber is installed at the centralized location. The multiplexer/selector is remotely controlled by a cable selector located at the subscriber location, and selects which of the plural distribution cables are coupled to the subscriber's single "drop" cable. The multiplexer/selector is also addressable by a "subscriber control system" for enabling/disabling service to subscribers on a subscriber-by-subscriber basis.